President Trump has named Assistant Treasury Secretary for Tax Policy David Kautter as acting commissioner of the IRS. He’d serve in both posts until a new commissioner is named. It is a terrible idea.

This is no criticism of Kautter, who has a strong resume as tax policy expert with years on Capitol Hill and as a tax practitioner. But choosing him to temporarily replace retiring IRS Commissioner John Koskinen while keeping his Treasury post is a mistake. He might be able to do either job well but he can’t do both.

Here are four reasons why:

Capacity. You may have noticed that Treasury is in the midst of an effort to write a major tax bill. The assistant secretary for tax policy should be playing a key role in that initiative, working with Congress to draft legislation and advocating for Administration priorities. And for many months, it will be a more than a full-time job. Yet Kautter is being asked to do that and run the IRS, another highly challenging post. If a tax bill slips into next year, a highly-probable result, and Trump can’t get a permanent commissioner confirmed, Kautter would have to both shepherd a major tax cut and manage a tax filing season. It is impossible to do both well.

Contrasting roles. The IRS Commissioner and the assistant secretary serve very different functions. The Commissioner is charged with administering the tax law as it is. The assistant secretary’s job is to determine what the law should be. These two perspectives can create a healthy tension that results in better law. But for the roles to complement one another, they should be fully independent.

The IRS needs a permanent commissioner. The agency faces enormous challenges. Its staff morale is poor, its technology is lagging, it faces near-constant political criticism, and it is desperately short of money. Trump has been talking about replacing Koskinen since long before election day and it is no surprise that the post will be vacant when Koskinen’s term expires in three weeks. Yet the White House is unprepared to name a replacement.

There have been acting commissioners in the past, but they are nearly always top IRS career officials who serve for a relatively brief time. And the Treasury says that two current deputy commissioners will run the day-to-day operations at the IRS. But the agency needs a top manager, an effective advocate with Congress, and a visible public face. Even in the best of circumstances, a part-time temp can’t serve all these roles.

The perception of politics. The IRS must…must…be perceived as non-political. Republicans, of course, have been complaining for years about the Service’s alleged bias. Yet, Trump is taking someone who is already serving in a highly political policy job and giving him the IRS post. The dual roles will inevitably raise serious questions about the independence of the IRS.

There is, I suppose, one benefit to Kautter serving dual roles. It will be easy for the IRS commissioner to convince the assistant secretary of the importance of tax administration in developing a new tax law. But other than that, Kautter has an important job that fits his skill set well—advising the White House on the design of a tax bill. Adding the IRS to his portfolio does an injustice to both him and the country.